Hydraulic fracturing of a subterranean formation is a fluid inflow stimulation technique wherein the fractures are artificially created in the subterranean formation of the field. The fractures are created by injecting the fracturing fluid with a pressure exceeding the fracturing pressure of the formation. The injected fluid bears a proppant (a propping agent) which is delivered into the fracture to prevent closing the fracture when the pressure is released upon completion of the inflow stimulation. An approach widely used for hydraulic fracturing is based on using high-viscosity fluids, such as cross-linked gels, as carrier fluids to transport the proppant from the surface through the wellbore and into the fractures formed in the course of hydraulic fracturing. The high viscosity fluid reduces the settling rate of the proppant particles resulting in a higher portion of the injected proppant being delivered to the intended location within the fractures, thus ensuring transportation of the particles and low rate of their settlement. But the use of this technology is limited, first of all, due to viscous fluids' inability to form a highly branched network of fractures in the formation. Hydraulic fracturing based on a low-viscosity fracturing fluid, for example, a friction-reducing aqueous polymer solution, is currently used more often in tight formations, such as shales or tight sands. Low viscosity of the fracturing fluid and/or high rate of its injection result in the formation of a highly branched network of fractures; however, the use of the low-viscosity fluids for hydraulic fracturing causes difficulties in preventing the proppant particles from settling due to poor transport properties of the carrier fluids. After the fracture closure, incomplete vertical covering of the fracture with the proppant is observed that is caused by the proppant settling. Existing techniques also present challenges in delivering the proppant into the secondary and tertiary (narrow) fractures of the complex fracture networks.
Patent application US20100252262 describes a method that includes fracturing fluid being injected into a formation under a pressure sufficient to create a fracture in the formation, wherein the fluid includes a proppant and gas to reduce the proppant settling rate. A composition that includes a proppant and microbubbles of gas in the amount (1 to 20% vol.) sufficient to reduce the proppant settling rate.
Patent application US20140060827 discloses an hydraulic fracturing method which includes blending the hydraulic fracturing fluid: carrier fluid with a predetermined density, proppant agglomerates with a density higher than the carrier fluid density, an additive of light particles (with a density lower than the carrier fluid density) and subsequent placement of the resultant hydraulic fracturing fluid into the fracture, wherein these light particles will slow down the proppant settling in the fracture. The proppant agglomerates are formed when the proppant particles mix with a consolidating (adhesive) agent.
A fluid composition with the injected gas (that is a foam) used for hydraulic fracturing is described in U.S. Pat. No. 7,932,214, wherein a composition based on an ionically coupled gel-like system is used as a viscous phase, which reduces the water consumption for the hydraulic fracturing operation. The “ionically coupled gel” claimed in U.S. Pat. No. 7,932,214 means a gel formed by the interaction between a charged polymer (polymeric electrolyte) and an oppositely charged surfactant. Such dense gel formed by the interaction between oppositely charged molecules (a charged polymer and a charged surfactant) possesses a high stability under downhole conditions. In the presence of foam stabilisers and lower alcohols such foamed composition demonstrates an increased viscosity and high stability of the foam, which allows the use of the described foamed gel system as a hydraulic fracturing fluid.
The analysis of the prior art has shown that there is a demand for an improved method of non-uniform/heterogeneous proppant placement in the extended and branched hydraulic fractures, which increases the fracture permeability compared to the conventional homogeneous proppant placement.
The description discloses a new approach to creating the proppant-bearing agglomerates (or proppant agglomerates) that are stable/strong during transportation through the wellbore and into the hydraulic fractures due to reduced losses of the proppant under shear loads to form the proppant structures that prevent fractures from closing. At the same time, due to the improved buoyancy of the agglomerates, which in turn is increased by improved carrying capability of the hydraulic fracturing fluids, their transfer and vertical placement of the proppant structures are improved. Moreover, the proposed technology reduces the hydraulic fracturing environmental effects due to the reduced amount of the carrier fluid to deliver the proppant into the fracture to the same depth.